In previous years we found that patients with winter seasonal affective disorder (SAD) had certain in vitro immunological abnormalities, which were restored to normality following effective light therapy. Subsequently we showed that bright light treatment increased the number of circulating CD4 (helper)-T cells in direct proportion to its antidepressant effect in a sample of the general population. Since HIV virus, among its many pathogenic effects, results in decreased numbers of circulating CD4 cells, we hypothesized that bright light exposure might increase the number of these cells in the circulation of HIV-infected individuals. To test the above hypothesis, we recruited 35 HIV-infected individuals, 17 with a history of SAD and 18 without such a history. We treated these individuals in a crossover study, using two different light sources for 45 minutes each morning for a total of four weeks. We predicted that the bright (10,000 lux) white light source would have an elevating effect on the CD4 cells, as compared with the dim (175 lux) red light source. Analysis of variance showed a significant effect of treatment on CD8 cell counts and a trend towards such an effect for CD4 counts. When the post- treatment results were compared directly, there was a small but statistically significant elevation in both CD4 and CD8 counts following the bright white treatment as compared with the dim red control. There was no difference between the SAD and the non-SAD groups with regard to their response to light. In addition, there was no evidence that the effects of light on immune cell levels was mediated by its effects on mood. The small effect size raises questions as to whether light might be of any value as an adjunctive treatment in HIV infected individuals. Studies of longer duration will be needed before this question can be answered. On the basis of the present study it appears that light treatment can be administered safely to HIV-infected individuals who need it for reasons unrelated to their infection.